Question: Simplify the following expression: $y = \dfrac{-5x^2+18x+35}{-5x - 7}$
Explanation: First use factoring by grouping to factor the expression in the numerator. This expression is in the form ${A}x^2 + {B}x + {C}$ First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(-5)}{(35)} &=& -175 \\ {a} + {b} &=& &=& {18} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $-175$ and add them together. Remember, since $-175$ is negative, one of the factors must be negative. The factors that add up to ${18}$ will be your ${a}$ and ${b}$ When ${a}$ is ${-7}$ and ${b}$ is ${25}$ $ \begin{eqnarray} {ab} &=& ({-7})({25}) &=& -175 \\ {a} + {b} &=& {-7} + {25} &=& 18 \end{eqnarray} $ Next, rewrite the expression as $({A}x^2 + {a}x) + ({b}x + {C})$ $ ({-5}x^2 {-7}x) + ({25}x +{35}) $ Factor out the common factors: $ x(-5x - 7) - 5(-5x - 7)$ Now factor out $(-5x - 7)$ $ (-5x - 7)(x - 5)$ The original expression can therefore be written: $ \dfrac{(-5x - 7)(x - 5)}{-5x - 7}$ We are dividing by $-5x - 7$ , so $-5x - 7 \neq 0$ Therefore, $x \neq -\frac{7}{5}$ This leaves us with $x - 5; x \neq -\frac{7}{5}$.